http://www.debian-administration.org/articles/409

To disable IPv6 it should be as simple as adding the lines

alias net-pf-10 off
alias ipv6 off

to /etc/modprobe.d/00local (creating the file if it exists).

It is important that this comes in a file earlier in an alphanumeric sort than aliases, since that contains aliases that turn this on.

Once you've made this change/addition you will need to restart the box afterwards - since the modules won't unload.

echo "install ipv6 /bin/true" > /etc/modprobe.conf

cat > /etc/modprobe.d/00_no_ipv6
blacklist ipv6

alias net-pf-10 off
alias ipv6 off

install ipv6 /bin/true
install sit0 /bin/true